Tube and other container



Patented Dec. 5, 1939 TUBE AND OTHER CONTAINER Harold D. Newell, BeaverFalls, Pa., assignor to The Babcock & Wilcox Tube Company, West Mayfield, Pa., a corporation of Pennsylvania No Drawing. Application May 11, 1938, Serial No. 207,380

3 Claims.

This invention relates to .improvements in tubes or other containers or parts thereof for use at elevated temperatures in heating fluids under high pressure. For oil refining or cracking or for the superheating of steam or for other similar purpose where fluids are heated under high pressures, high creep strength is required. Resistance to corrosion, as respects attack by both the heating gases and also the oil, steam or the like heated thereby, is also required for safe and economical operation of equipment.

Except for the highly alloyed austenitic steels of the chromium-nickel type, the prior art alloys do not possess high temperature strength prop erties suflicient for the ever increasing temperature and pressure demands of the oil refining industry, with the result that excessively heavy walled tubular bodies are required for existing operating conditions. This limits the volume of oil that can be handled per unit of time because of the decreased internal dimensions of the pipes, and the thick walls retard the flow of heat from the combustion gases to the contained liquids or gases. The heavy walls necessitated by present low strength steels gives rise to further disadvantages in increased external skin temperatures with consequent development of excessive temperature stresses in the walls of the heated tubes. These stresses may even exceed the stresses created by internal pressure and promote rapid failure with loss of life or property.

The corrosion resistance of alloy steels depends mainly on the chromium content. Chromium by itself, however, does not improve to any appreciable extent the high temperature creep strength of low carbonsteel, and it has been found that with the usual addition of 0.50% or thereabouts of molybdenum, the creep strength of the alloy is at the maximum with between 1 and 2% gchromium and decreases as the chromium content is either raised or lowered. The maximum corrosion resistance for minimum cost is ob- 'tained with about 2%,% chromium. Relatively little increase in corrosion resistance results from further small addition of chromium, and with reduction of chromium below about 2 the corrosion resistance falls off rapidly.

The high alloy chromiummickel steels, such as the low carbon 18% chromium and 8% nickel steel, have exceptional corrosion and oxidation resistance and great high temperature strength, but their high cost and certain disadvantageous physical characteristics have more or less limited their application to special uses. The straight chromium steels with upwards of 12% chromium become increasingly resistant to the sulphide corrosion occurring in oil refining processes as the percentage of chromium is increased, but not proportionally with the chromium increase, and they do not have the required strength prop- PATENT OFFICE erties and are lacking in structural stability. Steels containing about 5% chromium have been used to a considerable extent and have shown good resistance against sulphidic oils at high temperature, and a nominal addition of molyb- 5 denum, usually about 0.50%, has been made to such steels to reduce their tendency to cold embrittlement, adding, also, somewhat to their strength properties at high temperatures. Silicon has also been added to such steels for im- 1 proving their scale resistance at nominal cost, but tends to reduce the creep strength.

Molybdenum has been used in small amounts, usually not over about 0.60%, as a strengthening element in low carbon steels. Such addition im- 1 proves the creep value at high temperatures, but provides no improvement in resistance to corrosion from heated liquids or gases or to external scaling from gases of combustion. Molybdenum has also been added to the 5% chromium steels, 2 as stated, and to other comparatively low chromium steels, to reduce the tendency to temper embrittlement and to improve the comparatively low creep value at high temperature of the plain low chromium steels.

The object of the present invention is to provide tubes and other containers and parts thereof at comparatively moderate cost which, while having a sumciently high chromium content for good resistance to sulphidic oil or steam corrosion, have also very high resistanceto deformation under stress at elevated temperatures.

In my efforts to produce an alloy steel suitable for making such tubes and other containers, I have discovered, after an extensiveseries of tests, that the chromium content which gives the Highest creep strength at temperatures of about 1100 to 1200 F., instead of being below 2%, is between about 2 and about 3% when the molybdenum content is about 1 to 1.25%, and that when the chromium content is about 2.25% a molybdenum content of about 1 to 1.25% gives the highest creep strength at temperatures of about 1l00 to 1200 F.

I therefore make the tubes and containers of the'present invention of a low carbon alloy steel containing from about 2% chromium to not over about 3.25%, and from about 0.90% to about 1.75% of molybdenum. The carbon should be under 0.20% and preferably should not exceed about 0.15%. Manganese should not exceed about 0.50%, and phosphorous and sulphur should most desirably be kept low as in good alloy steel practice. When it is desirable to increase the oxidation resistance, silicon up to about 1 to 1.50% may be added, but such additions of silicon reduce the creep resistance and increase the difficulty of Working. For maximum creep strength properties, the silicon should not exceed about 0.60%. Aluminum deoxidation should most desirably be avoided, and if any aluminum is added for deoxidation when finishing the heat, the addition should be very small so that there is substantially no excess to alloy 5 with the metal, as even small proportions of aluminum in the finished product have a decidedly deleterious effect on the creep strength.

Thealloy steel is most desirably made by the electric melting practice to avoid loss of chromium and to insure a clean high quality metal adapted to be manufactured into seamless tubes by the conventional methods of cross-rolling. The'melting practice should be adjusted so as to obtain a medium coarse-grained type of steel. A McQuaid-Ehn or inherent grain size, under standard carburization test, between 3 and 6 as measured by the A. S. T. M. chart is preferable to the extremely fine grained 6-8 type or to the extremely coarse grained 1-3 type. The tubin treatment so designed as to give it an actual grain size of 3 to 6 A. S. T. M. and to place it in a softened ductile form for expanding or rolling into headers or tube sheets. In the as-rolled condition, the alloy is hard and of comparatively low ductility due to the air-hardening action induced by chromium. The alloy steel is of the pearlitic type.

A preferred alloy steel according to the invention has approximately the following analysis:

Per cent Carb n v 0.12 Chromium 2.25

- Molybdenum- 1.05 Manganese 0.35 Silicon 0.50 Iron and usual impurities Balance Such an alloy has a creep strength of about 9,000 0 pounds at 1100 F. and 4,200 pounds at 1200 F.

for a creep of 1% in 10,000 hours. Another preferred alloy steel according to the invention has approximately the following analysis:

Per cent Carb 0.12 Chromium 3 Molybden 1 Manganese 0.45

Silicon 0.45

Iron and usual impurities Balance Such an alloy has a creep strength approximately so produced may be subjected to an annealing equal to that of the 2.25% chromium and 1.05%

molybdenum alloy.

The present invention makes it possible to reduce the wall thickness of tubes and other parts formerly made of materials of comparable cost, or to provide additional strength so that higher temperatures and pressures can be employed in refinery processes with consequent increase in yield and improvement in quality of product. The invention increases factors of safety in refinery heaters and other fluid heating apparatus which have to Withstand high temperatures and high pressures, because of the greater strength of the tubes or other parts at high temperature and because of their good corrosion resistant qualities necessary to long life and economical operation. The outstanding characteristic of the tubes and containers according to the invention is that they have a creep strength at high temperatures much in excess of the pearlitic type steel tubes and containers heretofore available for high temperature service within the operating temperature range of from about 1000 F. to about 1250 F.

The expression elevated temperatures as used in the claims is to be understood as meaning temperatures of the order of from 1000 to 1300 F.

What is claimed is:

1. A tube or other container or part thereof for use at elevated temperatures in heating fluids under high pressure having high creep strength and corrosion-resistance properties, made of an alloy containing about 2 to 3.25% chromium, about 0.9 to 1.75% molybdenum, and carbon below 0.20%, the balance being substantially iron.

.2. A tube or other container or part thereof for use at elevated temperatures in heating fluids under high pressure having high creep strength and corrosion-resistance properties, made of an alloy containing about 2.25% chromium, about 1% molybdenum, and not more than 0.15% carbon, the balance being substantially iron.

3. A tube or other container or part thereof for use at elevated temperatures in heating fluids under high pressure having high creep strength and corrosion-resistance properties, made of an alloy containing about 3% chromium, about 1% molybdenum, and not more than 0.15% carbon,

the balance being substantially iron.

HAROLD D. NEWELL. 

